Unable to unlock GoW3 achievement.

I am in need of assistance with an issue concerning the achievement "We Few, We Happy Few". I have completed this achievement 3 times to date and even show all required check marks and have all the correct icons for each chapter that tells me I did complete
it, yet the achievement will not unlock nor will the associated mutator. I have already attempted the profile deletion and recovery from the console numerous times and this has not resolved the issue. I have played through the campaign multiple times with
the correct/required number of players and this too has not resolved the issue. I need help getting this achievement unlocked as I am not able to unlock the 50 gamer score or mutator nor am I able to get game completion. I would appreciated it if someone could
look into this. I have already contacted support and they have not been able to help thus far.